1. Field of the Invention
The present invention relates to a connecting board disposed between a base plate having surface-bonding pads, such as a LGA type integrated circuit package base, and a mounting board such as a motherboard, to bond or connect the base plate to the mounting board. The present invention further relates to a method of making such a connecting board. The present invention further relates to an assembly of a base plate, connecting board and mounting board, and a subassembly of a base plate and connecting board. The present invention further relates to a method of making such an assembly and a method of making such a subassembly. The present invention further relates to a method of disassembling such an assembly.
2. Description of the Related Art
Recent development of the integrated circuit (IC) technology has brought about a continuous increase in the number of the input/output terminals provided to each IC chip. However, in case the input/output terminals are arranged at the peripheral portion of an IC chip, the size of the IC chip is increased with increase of the number of the input/output terminals, so it results undesirably that the cost of the IC chip and an IC chip package base is elevated and the yield rate of same is lowered.
Thus, it has been widely used a so-called flip chip technology, i.e., a technology by which a plurality of bumps are disposed on a main surface (planar surface) of an IC chip so as to form a grid pattern or checkered pattern, while a plurality of pads are disposed on a base plate at corresponding positions to the bumps of the IC chip and the both (i.e., bumps and pads) are soldered or joined together all at once. Further, in case of bonding a base plate having mounted thereon an IC chip or the like to a printed circuit board (PCB) such a motherboard, it is also carried out that a plurality of pads are disposed on the surface of the motherboard substrate to be matched with the base plate, a plurality of bumps having previously soldered thereto nearly spherical (ball-shaped) terminal members made of high temperature solder or Cu are provided to either of the base plate or the motherboard, and thereafter the both (i.e., the base plate and motherboard) are bonded together by soldering by way of the terminal members. In general, the base plate having only pads which are disposed so as to form a grid pattern is called a LGA (land grid array) base plate and a base plate having ball-shaped terminal members (connecting terminals) on pads is called a BGA (ball grid array) base plate.
In the meantime, in case pads and terminals are formed in the above manner on the surfaces of the IC chip, base plate and motherboard so as to form a linear pattern or grid pattern (including a checkered pattern) for bonding the IC chip and the base plate together and for bonding the base plate and the motherboard (hereinafter such bonding being referred to as "surface bonding") together, a difference in thermal expansion is caused between the IC chip and the base plate and between the base plate and the motherboard substrate along the mating planar surfaces thereof because they differ in material and therefore in coefficient of thermal expansion. In other words, the IC chip-mounted base plate and the motherboard substrate which are bonded together by way of the terminals are caused to deform in the same directions along the matching planar surfaces thereof but by a different amount from each other so that shearing stress is caused in the terminal members.
This shearing stress becomes maximum between the two terminals which are disposed remotest from each other when the magnitude of the shearing stress is taken into account between any two surface-bonded terminals. More specifically, in case the terminals are arranged so as to form a grid pattern and the outermost terminals form a square, the largest difference in thermal expansion occurs between the outermost terminals located at each pair of diagonal corners of the square, and the largest shearing stress is caused on each pair of the outermost terminals. Particularly, when a base plate such as a LGA type base plate or a BGA type base plate is bonded to a motherboard substrate, the interval (pitch) between the terminals is relatively large, so the distance between the most remotely separated terminals tends to become large. Further, when a LGA type or BGA type base plate is made of ceramic, the base plate has a smaller coefficient of thermal expansion than a motherboard substrate which is generally made of glass epoxy resin, so the resulting shearing stress is large.
When such shearing stress is applied to the joint between the base plate and the motherboard substrate, there may occur such a case in which a thermal stress is applied repeatedly to the masses of solder which are bonded to the pads of the base plate to cause a crack or cracks in some of the masses of solder at portions thereof adjacent the joint between the masses of solder and the pads, so it has been incapable of obtaining a high reliability.
In the meantime, by Japanese patent provisional publication No. 8-55930, it is disclosed a package for housing semiconductors, having pads disposed on the bottom of the depression formed in the lower surface of an insulation substrate and ball-shaped terminal members having a predetermined dimensional relation to the pads and soldered to the pads. It is also disclosed that by this arrangement the ball-shaped terminals can be brazed to the pads accurately and assuredly.
In the package of this prior art, however, depressions must be formed in the insulation substrate (base plate) and pads must be provided to the bottom of the depression, leading to a complicated shape, a difficulty in manufacture and an increase in cost.
Further, IC chip makers or manufacturers, after having purchased LGA type base plate on which IC chips are to be mounted, having mounted the IC chips on the base plate and having performed flip chip bonding, are required to bond the terminal members to the pads (lands) of the base plate by means of low temperature solder (for example, eutectic solder) which has a melting point lower than the solder (for example, high temperature solder) used for the flip chip bonding. In addition to the apparatus for flip chip bonding the IC chips to the base plate, there are required apparatuses for fabricating a BGA type base plate from a LGA type base plate, i.e., an apparatus for applying solder paste (for example, eutectic solder paste) to the pads and an apparatus for mounting the terminal members on the pads.
Further, by IC chip users (end users), it is carried out a difficult process for bonding the BGA type base plate to a printed circuit board, i.e., a process in which low temperature solder is applied to pads on a printed circuit board, a BGA type base plate is placed on the printed circuit board, the terminal members are aligned with the pads of the printed circuit board, and thereafter reflowing is carried out to bond by soldering the pads of the printed circuit board to the terminals.
Accordingly, the end users are required to apply low temperature solder to the pads of the printed circuit board, place the BGA type base plate on the printed circuit board, and put the printed circuit board and so on into a reflow furnace to bond the BGA type base plate to the printed circuit board.
Moreover, When it is found, after the BGA type base plate and the printed circuit board have been bonded in the above manner, that the mounted IC chip, the base plate or the printed circuit board is defective, the printed circuit board is removed from the BGA type base plate. This work is further difficult. This is because removal of the BGA type base plate from the printed circuit board to which the BGA type base plate have been connected by surface bonding requires to melt the low temperature solder by which the both (i.e., BGA type base plate and printed circuit board) are bonded. However, in consideration of the heat-resistant temperature or property of the printed circuit board, the heat-resistant temperature or property of the BGA type base plate and the IC chip, the heat-resistant temperature or property (melting point) of the solder in the flip-chip bonding of the pads of the IC chip and the BGA type or LGA type base plate, an allowable temperature range in heating is limited to be so narrow.
Furthermore, if it is tried to remove the BGA type base plate from the printed circuit board before some of the masses of solder (for example, several masses of solder) have not yet been melted, there may result such a case in which the pads of the BGA type base plate or the printed circuit board are damaged, and they finally become not reusable in not a few number of cases. Further, even if they are separated successfully, such separation cannot be always done in a way that all of the terminal members such as high temperature solder balls are in the same condition as they have assumed when they have been bonded to the LGA type base plate, i.e., when the BGA type base plate have been formed, so in case some terminal members remain on the printed circuit board side, it is further required to remove such terminals and bond them in place once again. Thus, it is difficult or it results in a poor reworking ability to separate the LGA type base plate (or BGA type base plate) and the printed circuit board having been bonded, from each other and to bond them once again.
Further, some of the printed circuit boards such as a motherboard or the like are adapted to mount thereon a plurality of BGA type base plates. When this is the case, the printed circuit board and a plurality of BGA type base plates are needed to be bonded (soldered) together at one time. If the printed circuit board having already bonded thereto one of the BGA type base plates is put into a reflow furnace for bonding thereto another BGA type base plate, even the solder between the printed circuit board and the BGA type base plate having already been bonded to the printed circuit board is caused to melt, so there is a possibility that the base plate having previously bonded to the printed circuit board is moved out of place.
In case of removing only particular one of BGA type base plates having mounted on a printed circuit board, it is necessary to consider the influence to the other base plates in addition to the above described difficulty and therefore a still more difficult work is necessitated.